Telephone system



WICKS ET AL TELEPHONE SYSTEM Filed May 22, 1947 is Sheets-Sheet 4 C392 (3393 REPEATER CIR. 400

Ls 42 RA43 5437 442 55 RLS. PULSE TIMER I 477 $556 5O SEIZE INVENTOR. 4 John Wicks Orho D. Grun'dsmlff Aug. 1, 1950 J. WICKS ETAL 2,517,

TELEPHONE SYSTEM Filed May 22, 1947 l5 Sheets-Sheet 5 LOOP REPEATER 400 FALSE SEIZURE INVENTOR. John Wicks FIG. 5 Q y Oiho 0. Grundstoff W 06w" 4 M Attorneys g- 1950 J. WICKS EI'AL 2,517,516

TELEPHONE SYSTEM Filed May 22, 1947 15 Sheets-Sheet 6 REPEATER CIR. 400

INTER-PULSE IMER 1 -TONE UNIT- INVENTOR. H 6 SUPY'PULSE John Wicks BY Oiho D.Gr0nds1uff 1950 J. WICKS EI'AL 2,517,516

TELEPHONE SYSTEM Filed May 22, 1947 15 Sheets-Sheet 7 REPEATER CIR. 70o

702 c70 +REC.

Voice Current Repeaiers 703 R730 SEND i:

ILA/Z44 PAD CONT.

TIME PULSE lnvenfors John Wicks Orho D. Grondsroff M I M Attornevs Aug. 1, 1950 J. WICKS HAL 2,517,515

TELEPHONE SYSTEM 8'3 iesi CIIO Inventors CHO John Wicks Ofho D. Grondsfrcff y M,0CQW+M Arror neys Aug. 1, 1950 Filed May 22, 1947 FIG. 9

J. WICKS ET! AL 2,517,516

TELEPHONE SYSTEM 15 Sheets-Sheet 9 FINDER MON. STOP R990 Inventors By John Wicks Otho D. Grondsfoff 1950 J. WICKS EI'AL 2,517,516

TELEPHONE SYSTEM Filed May 22, 1947 15 Sheets-Sheet" l1 y Oiho D. Gronds'roff M. 06% PM Aflorneys Patented Aug. 1, 1950 TELEPHONE SYSTEM John Wicks, Biloxi, Miss., and Otho D. Grandstaff,

Oak Park, 111., assignors to Automatic Electric Laboratories, Inc., Chi

Delaware cago, 111., a corporation of Application May 22, 1947, Serial No. 749,758

The present invention relates to telephone systems and more particularly to toll switching systems of the type disclosed in U. S. Patent No. 2,155,176, John Wicks, granted April 18, 1939.

In the toll switching system disclosed in the wicks patent, the toll lines inter-connecting the different exchanges are terminated by repater circuits, each of which is provided with a tone unit individual thereto of the connection and arrangement disclosed in U. S. Patent No. 2,159,081, Otho D. Grandstaff, granted May 23, 1939. Each of these tone units comprises both a transmitter and a receiver of unmodulated audio frequency signals and fixed frequency modulated audio frequency carrier signals that are employed for the purposes of digit control, seizure control, ring control, supervisory control and release control.

In the present toll switching system, improved repeater circuits are provided, each of which has individually associated therewith a tone unit of the improved connection and arrangement disclosed in the copending application of Otho D. Grandstaff, Serial No. 738,194, filed March 29,

1947, now U. 5. Patent No. 2,482,478, issued Sopand operating facilities in order to accommodate the handling with ease of heavier toll traffic b the operators at the various toll switchboards in the different exchanges in the system.

Another object of the invention is to provide in a telephone system of the type noted, a trunk circuit comprising improved monitoring facilities.

Another object of the invention is to provide in a telephone system of the type noted, repeater circuits interconnected by a line and comprising improved arrangements for insuring release of both of the repeater circuits in the event either one of them is falsely seized over the associated line.

Another object of the invention is to provide in a telephone system of the type noted, repeater circuits interconnected by a line and comprising improved arrangements for attempting the repeated release of one of the repeater circuits by the other of the repeater circuits in the event the one repeater circuit fails to release upon any preceding attempt.

Further features of the invention pertain to the particular arrangement of the circuit elements of the system, whereby the above-out- 20 Claims. (01.179-16) lined and additional operating features thereof are attained.

The 1 general arrangement of the telephone -systcm The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the following specification taken in connectionwith the accompanying drawings in which Figures 1 and 2 taken together, when arranged in end-to-end relation with Fig. 1 on the left and Fig. 2 on the right, constitute a schematic diagram of the area served by a telephone system embodying the present invention and illustrate the general arrangement of the apparatus incorporated in several of the exchanges and offices thereof; Figs. 3 to 14, inclusive, taken together illustrate the details of certain of the apparatus incorporated in two of the remotely disposed automatic exchanges included in the telephone system, which apparatus has incorporated therein the features of the invention as briefly outlined above; Fig. 15 illustrates the mode of combining Figs. 3 to 14, inclusive, to form a unified diagram; and Fig. 16 illustrates the details of one of the tone units incorporated in the apparatus illustrated in Figs. 3 to 14, in-

elusive.

More particularly, Figs. 4 to 6, inclusive, illus trate the details of one of the repeater circuits incorporated in one of the automatic exchanges; Figs. 7 to 13, inclusive, illustrate the details of one of the repeater circuits incorporated in another of the automatic exchanges; and Fig. 14 illustrates a fragmentary portion of one of the coders associated with the repeater circuit illustrated in Figs. '7 to 13, inclusive.

Referring now more particularly to Figs. 1 and 2, the telephone system there illustrated was one applied to meet the specific requirements of the system of toll lines inter-connecting a group of cities in Brazil, South America, and including the cities of Sao Paulo and Rio de Janeiro. Since these two cities comprise the principal cities of Brazil, each serves as a concentration point for toll traflic; and as these two cities are within about two hundred miles of each other considerable toll trafiic passes therebetween. In the general area of Sao Paulo are a number of cities, including Santos, Campinas and Jundiahy; and in the general area of Rio de J aneiro are a number of cities, including Campos. Sao Paulo comprises anautomaticexchange 3c constituting a toll center; and a number of automatic oflices,

corporated in the automatic exchange 33, con-.

stituting a toll center, as well as the switching apparatus incorporated in the automatic oflice 383, is of the well known Strowger type. Santos comprises an automatic exchange 360 serving a group of subscriber substations, including the subscriber substation T2, and automatic switching apparatus of the well known Strowger type. Campinas comprises an automatic exchange 3'53 serving a group of subscriber substations, including the subscriber substation T3, and automatic switching apparatus of the well known Strowger type. Jundiahy comprises a manual exchange 38! and serves a group of subscriber substations. not shown.

Rio Janeiro comprises an automatic exchange constituting both a toll center and an ofhce servin a grouoof subscriber substations, including the subscriber substation T4. Also, the automatic exchange 53 comprises automatic switching apparatus of the well known International Standard Electric 'T-A type, utilizing revertive impulse control. Campos comprises a manual exchange 6!} and servesa group of sub-- scrih'er substations, not shown.

The automatic exchange iiuis connected to the automatic exchange 593 by a; group of two-Way toll lines, including thetollline N32. The toll line M9, embodies a number of voice current'repeaters- H 3 distributed therealong; and the opposite'ends thereof are terminated in repeater circuits 403 and TM, respectively disposed in the automatic exchange 33 and inthe: automatic exchange 50. A tone unit 35! in the automatic exchange 30 is individually associated with the repeater circuit and a tone unit ml in the-automatic exchange 53' is individually associated with the repeater circuit 109.

The automatic exchange30 is'connecte'd-to the automatic exchange 333 by a group of two-way tolllines, including the toll line 356. The" toll line 353 embodies a number of voicecurrent repeaters distributed therealong; and the oppo site ends thereof are terminated in repeater circuits and 3'55. respectively disposed in the automatic exchange 33 and in the' automatic exchange 333. A tone unit 358 in the automatic exchange 33 is individually associated with the repea er circuit'355 and a tone-unit 363m the auto-- matic exchange3tfl is individually associated with the repeater circuit 365.

, The automatic exchangetl] is connected to the automatic exchange 3'1!) by a group of two-way toll lines, including the toll. line 35 l. The toll line 35 embodies a number of voice current repeatersdistributed therealong; and the opposite ends thereof are terminated in repeater circuits 350 3'55, respectively disposed in the automatic exchange 33 and in the automatic exchange 310. A tone unit 353 in the automatic exchange 30 is individually associated with the repeater circuit 3'53 and tone unit 3'i8'in the automatic exchange 3'53 is individually associated with the repeater circuit 375.

The automatic exchange 30 is connected to the manual exchange 303 by a group of two-way toll lines, including the toll line 302. One end of the toll line 332 terminates in a manual toll board 33! disposed in the manual exchange 300, and the 1 a trunk Bill to the switch train 385;

other end of the toll line 332 terminates in atoll trunk circuit 3m disposed in the automatic exchange 33.

The automatic exchange 33 is connected to the automatic ofiice 383 by a group of outgoing trunk lines, including the outgoing line 342, and by a group of incoming trunk lines, including the incoming trunk line 3363. The outgoing trunk line 332 is accessible to the automatic switching apparatus in the automatic exchange 30 and terminates-in the switch train 385 disposed in the automatic oflice 335. One end of the incoming trunk line 333 terminates in a trunk circuit 388 in the automatic office 335! and is accessible via and the other end of the incoming trunk line 335 terminates in a: CLR trunk circuit 363 in the automatic exchange 33. In passing, it is noted that the switch train in the automatic office 383 has access to and is accessible to the subscriber line 385 extending. to the subscriber substation TI.

The automatic exchange 5t-is connected to the manual exchange til by way of a group of twoway toll lines; including the toll line 535i. One end. of the toll line I353: terminates at a manual toll board disposed in the manual exchange 30-, and the other end of the toll line l36l tor-- minates in a toll trunk circuit I311 disposed in the automatic exchange 50.

The automatic exchange 33 further comprisesa strowge'r dial toll board, the first operator position circuit of which is indicated at 316; which first operator position circuit 3H5 has access by way of an associated cable 3% to a number of finder selector links, including the finder-selector link 315. a rotary finder having access to trunks 3M and 33?, respectively extending to the CLR trunk circuit 335' and to the toll trunk circuit 3H], and a Strowger selector. Individually associated with the repeater circuits 353, 4-30 and 355, respectively terminating the toll lines 35 I, 102 and 355 in the automatic exchange 33, are trunks 326, 390 and 331 respectivelyterminating in incoming toll selectors 325, 323 and 33!). Also, the automatic switching apparatus in the automatic exchange 33 comprises a first group of outgoing toll selectors, including the outgoing toll selector 335, a second group of outgoing toll selectors, including the Outgoing toll selector 343, and a third group of outgoing toll selectors, including the outgoing toll selector 345. The selector of each of the finder-selector links 3I5, etc., and each of the incoming toll selectors 325, 32 b, 333, etc., has access to each of the outgoing toll selectors 335, 333, 335, etc., via respectively associated trunks 333, 33!, 343, etc. The outgoing toll selector 335, etc, access to a trunk 33! extending to the toll trunk circuit 313 and, consequently, to the toll line 333 extending between the automatic exchange 33 and the manual exchange 333. The outgoing toll selector 333, etc., has access to the outgoing trunk line 333 extending to the switch train 385 in the automatic office 388. The outgoing toll selector 335, etc., has access to trunks 333, 395 and 39'! respectively extending to the repeater circuits 353, G33 and 355; and, consequently, to the respective toll lines 35!, 102 and 356 extending between the manual exchange 33!! and the respective automatic exchanges 310, 50 and 360'.

The automatic exchange 35!] further comprises-a Strowger dial toll board indicated at 359, and provided with automatic switching apparatlls and control equipment indicated at 351.

The findenselector link 3l5 comprises The automatic switching apparatus 361 has access to and is accessible to the subscriber line extending to the subscriber substation T2. Also, theswitching apparatus 351 is accessible to and has access to the repeater circuit 305 via connections 335; and, consequently, with respect to the toll line 355 extending between the automatic exchange 36!) and the automatic exchange 30. The automatic exchange 330 further comprises a Strowger dial toll board indicated at 319, and provided with automatic switching apparatus and control equipment indicated at 371. The automatic switch apparatus 311 has access to and is accessible to the subscriber line extending to the subscriber substation T3. Also, the switching apparatus 33'! has access to and is accessible t'o'the repeater circuit 315 via connections 3l'i;.

and,consequently, with respect to the toll line 35l extending between the automatic exchange 310 and the automatic exchange 30.

The automatic exchange 50 further comprises a manual dial toll board of the well known West ern Electric cloiupany No. 3 type, the first inward operator position circuit of which is indicated at l3lfi, the first outward operator position circuitof which is indicated at I340, and the first transfer operator position circuit of which is indicated at I353. The first inward operator position circuit lfile is provided with an associated group of cord circuits, one of which is indicated at I 3I0, and provided with an answer plug IMI and a call plug I3I2. Further, the first outward operator position circuit I300 and the first transfer operator position circuit I350 are pro vided with associated groups of cord circuits, not shown, eachterminating in answer and call plugs. Also, associated with the manual dial toll board is a display panel I336 that is connected to the repeater circuit I00, etc., in order to display supervisory signals from the repeater circuit 100, etc. Further the repeater circuit is provided with two trunks I370 and I380. The trunk I370 is an incoming trunk and extends to the automatic switch train I300 and is accessible thereto; while the trunk I380 is a two-way trunk and terminates in 553M, JII and JII', respectively disposed at the first inward operator position circuit iiilii, at the first outward operator position circuit I340, and at the first transfer operatorposition circuit I350, disposed at the manual dial toll board in the automatic ex-,

change 50. Also, the automatic switch train I300 has access to and is accessible to thesubscriber line I30! extending to the subscriber substation T4. has access to a group of outgoing trunks, in-

cluding the outgoing trunk i352, and is acces-- sible to a group of incoming trunks, including the incoming trunk i363. The outgoing trunk I362 is terminated in a CLR trunk circuit I302 and the incoming trunk I333 terminates in a toll trunk circuit 5303. The toll trunk circuit I303, etc., the CLR trunk circuit I302, etc., and the toll trunk circuit I3I'l, etc., respectively terminate in jacks M303, M309 and JI3I3 disposed at the first inward operator position circuit I356 and multipled to similar jacks, not shown, disposed at the first outward operator position circuit I300 and at the first transfer operator position circuit @350.

Further, a group of toll register senders, including the toll register sender IBM, and a group of local register senders, including the local register sender I392, are commonly associated with the automatic switch train I300 in the automatic Further, the automatic switch train I300 exchange 50. Further, in the automatic exchange 50 there is disposed a preselector I403 having. access We group of coders, including the coder M04, and to the repeater circuit I00, etc. Specifically, the preselector I003 has access to the coder I404 via a cable M20 and access to the repeater circuit #00 via a cable Mill. Also, the automaticexchange 50 comprises a group of finder circuits, including the finder circuit Mill, and an associated group of register senders, including the register sender M02,as well as an associated allotter M05. The allotter commonly associated withrepeater circuit I00, etc., and with the finder circuit I40I, etc.; while the finder circuit I40I', etc., has access to the repeater circuit 700, etc., via a cable I090, etc. Finally, the finder circuit I404 is individually associated with the register sender I402.

The apparatus incorporated in the telephone system In the automatic ofiice 380 the switch train 385 is of the well known Strowger type, as previously noted, while the trunk circuit 388 may be of any suitable type.

r control equipment disposed in the automatic exchange 30. Further, the repeater circuit 365 disposed in the automatic exchange 360 is identical to the repeater circuit 355 disposed in the automatic exchange 30; which, in turn, is identical to the repeater circuit 400 described in detail hereinafter.

to the tone unit 353 disposed in the automatic exchange 30; which, in turn, is identical to the tone unit 6000 described in detail hereinafter.

In the automatic exchange 3l'0 the toll switchboard 313 is of the well known Strowger dial type, as previously noted, and may be identical to the Strow ger dial toll board provided in the automatic exchange 30. Similarly, the automatic switch and control equipment 31! disposed in the automatic exchange 310 is of the well known Strcwger type, as previously noted, and may be identical to the corresponding automatic switch and control equipment disposed in the automatic exchange Further, the repeater circuit 315 disposed in the automatic exchange 310 is identical to the repeater circuit 350 disposed in the automatic exchange 30; which, in turn, is identical to the repeater circuit 400 described in detail hereinafter. Finally, the tone unit 3'78 disposed in the automatic exchange 310 is identical to the tone unit 353 disposed in the automatic exchange 35; which, in turn, is identical to the tone unit I300 described in detail hereinafter.

In the automatic exchange 30, preferably the CLR trunk circuits 300, etc., are identical and are of the form illustrated in Fig. 3 of the previously mentioned Wicks patent; and the toll trunk circuits 350, etc., are identical and are of the form illustrated in 4 of the previously mentioned Wicks patent. Also, in the automatic exchange 30 the various operator position circuits 3l6, etc., at the Strowger dial toll board therein are identical and areofthe form illustrated inFig.

Finally, the tone unit 368 disposed in the automatic exchange 300 is identical "of the previously mentioned Wicks patent. Similarly, the finder-selector links 315, etc., are identical and are preferably of the form illustrated in Figs. 5 and 6 of the previously mentioned Wicks patent. Preferably, the incoming toll selectors 325, 3211, 330, etc., are identical and are of the form illustrated in Fig. 6 of the previously mentioned Wicks patent; the outgoing toll selectors 345, etc., are identical and are of the form illustrated in Fig. 8 of the previously mentioned Wicks patent; the outgoing toll selectors 340, etc., are identical and are of the form illustrated in Fig. 9 of the previously mentioned Wicks patent; and the outgoing toll selectors 335, etc., are identical and are of the form illustrated in Fig. 10 of the previously mentioned Wicks patent. .As previously noted, the repeater cir cuits 550, 355, etc., are identical to the repeater circuit 400 illustrated in Figs. 4 to 6, inclusive, and described in detail hereinafter; while the tone units 353, 601, 358, etc., are identical to the tone unit 1500 illustrated in Fig. 1:6 and described iii-detail hereinafter.

In the automatic exchange 50 the automatic switch train 1300 is of the International Standard Electric 'l-A type, as previously noted; the manual dial toll board is of the well knoWnWestern Electric Company No. 3 type as previously noted, cord circuit 1310 and the position circuit 13% being of the forms respectively in Figs. 2c and 21 of. the previously mentioned Wicks patent. Preferably,

he toll register senders 1391, etc., and the local register senders 1392, etc., are of the usual types; and the toll trunk circuits 1303, etc., the CLR trunk circuits 1302, etc., and the toll trunk circults 5311, etc., maybe of any suitable types. Further, the tone units 101, etc. are identical to the tone unit 1600, illustrated in 16 and described iii-detail hereinafter. Preferably, the repeater circuits disposed in the automatic ex change 5% are identical to the repeater circuit illustrated in Figs. 7 to 13, inclusive, and described in detail hereinafter. Preferably, the preselector 1 105 is of the form illustrated in Fig. 15 of the previously mentioned Wicks patent; and the coders 1404, etc., are identical and, as indicated in Fig. 14, are substantially of the form illustrated in Fig. 16 (parts 1, 2 and 3') of the previously mentioned Wicks patent. Preferably, the finder circuits 1401, etc., are identical and are of the form illustrated in Fig. 17 of the previously mention d Wicks patent; the register senders 1400, etc., are identical and are of the form illustrated in 18 (parts 1 and 2) of the previously mentioned Wicks patent; and the allotter 1405 is of the form illustrated in Fig. 17 of the previously mentioned Wicks patent.

The repeater circuit 0 disposed in the automatic exchange 30 comprises, as shown in Figs. 4: to 6, inclusive, a relay group including a series relay R-l i ii, a release relay R4 2'0, a line relay R430, two release pulse timer relays R435 and R440, a

selector seize relay R450, an incoming release relay R560, a tandem test relay R410, a line supervisory relay R480, an answer supervisory relay R590, a relay R l lfi, an answer supervisory relay R510, a supervisory control relay R520, a supervisory relay R530, a ring relay R535, three ring control relays R540, R550 and R560, a switchover relay Rli'l'il, a dial-in relay R560, a control relay R590, a loop test relay R500, a control relay R5135, a supervisory relay R610, a signa1-i-n relay an interpulse timer relay a switcln through relay R640, a guard relay R650, a supervisory pulse relay R660, a prepare relay R610, a

send relay R615, a sequence relay R680, .a padcontrol relay R690 and a pad cutout relay R600, as well as a toll pad 695.

The repeater circuit '10s disposed in the automatic exchange comprises, as shown in Figs. '7 to 13, inclusive, a relay group including a pad cutout relay R110, .a pad control relay R120, 9. send relay Rliil, a switch-through relay R140, a prepare relay R150, a cutoff relay R160, a time pulse relay Rl'lfl, an auxiliary relay R180, a rerin-g relay R810, a control relay R020, an end-ofdial relay R835, a supervisory relay R840, a. supervisory start relay R850, a loop test relay R860, a signal-in relay R010, a dial-in relay R880, a selector seize relay R890, a switch-over relay Rdlil, an incoming release relay R920, a control relay R030, a coder seizing relay R040, a control relay R950, a ring relay R560, a sequence relay Rdlil, a finder stop relay R800, a monitor relay R990, a release pulse timer relay R1010, an answer relay R1020, a release pulse timer relay R1033, a supervisory relay R1048, a series relay R1950, a line relay R1060, a sleeve relay R1010, a start relay R1080, two transfer relays R1110 and R1120, a coder supervisor relay R1130, a register sender supervisory relay R1140, a control relay R1 150, a flash relay R1 100, two re-ring relays R1 R13 and Rl210, a sleeve relay R1220, a cutoff relay areas, a signal relay R1240, a position transfer relay R5256 and two switch release relays R8256 R1216, as well as a toll pad 195. As previously noted, the coder 1404 disposed in the automatic exchange 50, shown in skeleton diagram in Fig. 14, is substantially of the form illustrated in Fig. 16 (parts 1, 2 and 3) of the previously mentioned Wicks patent; however, the coder 1404- is slightly modified in that it includes an additional relay, the transfer control relay R1430, which is utilized in carrying out an additional feature incorporated in the coder 1404 and explained more fully hereinafter. Otherwise, the coder is of the form mentioned and comprises the six identical digit storage devices DSI to DSB, inclusive; the digit storage device DSI being illustrated as including the four individual relays A, B, C and D.

The tone unit 1600 is of the type disclosed in the previously mentioned copending application of Otho'D. Grandstaff, and essentially comprises, as illustrated in Fig. 16, a receiving transformer TRl a sending and coupling transformer TRZ, a monitoring transformer TR3, a two-way amplifier T1 in the form of a vacuum tube of the well known pentode 6C6 type, a differential detector 1' T2 in the form of a vacuum tube of the well known dual diode 6H6 type, an impulse amplifier T3 in the form of a vacuum tube of the well known pentode 5C6 type and a signal amplifier T4 in the form of a vacuum tube of the well known pentode 43 type, as Well as a relay group including a modulation cutoff relay R1620, a receiving switch relay R1640, a pulse cut-in relay R1050, an alarm relay R1655, an impulse-in relay R1660, and a signal-in relay RIG'ld together with a plate impedance 1628 associated with the tube T1, a plate impedance 1634 associated with the tube T3, and a plate impedance 1639 associated with the tube T4. Further, the tone unit 1600 comprises circuit elements described in detail hereinafter and inter-connecting the elements mentioned, as Well as a group of conductors extending to the associated repeater circuit, and including a sending conductor (316 1, a sending conductor C1602, a monitoring conductor C1603, a monitoring conductor C1604, a

receiving conductor CI605,a receiving conductor CI606, a receiving switch conductor CI601, a modulation cutoff conductor CI608, a signal-in conductor CI609, a dial-in conductor CI6I0, and a pulse cut-in conductor CI6II; together with a 60 cycle conductor CI6I2 extending to an associated 60 cycle generator, not shown, and a 1000 cycle conductor CI6I3 extending to an associated 1000 cycle generator, not shown.

The operation of the tone unit 1600 In the tone unit I600, the tubes TI and T3 are each provided with a cathode heater having a nominal rating of 6.3 volts. at which voltage 0.3 ampere is drawn; the tube T2 is provided with a two-section cathode heater having a nominal rating of 6.3 volts, at which voltage 0.3 ampere is drawn; and the tube T4 isprovided with a cathode heater having a nominal rating of 25 volts, at which voltage 0.3 ampere is drawn. The cathode heaters of the tubes TI, T2, T3, and T4 are connected in series circuit with a 4 ohm resistor RI and the winding of the alarm relay RI655 across a 48 volt. exchange battery, the positive pole of which is grounded. More particularly, the circuit mentioned extends from ground by way of the contacts I6 I 9 of a normally operated key K2, the cathode heater of the tube T2, a conductor Cl 613, the cathode heater of the tube T3, a conductorCI6'I4, the cathode heater of the tube TI, a conductor CI6I5, the resistor RI, a conductor CIB'IG, the cathode heater of the tube T4, and the winding of the alarm relay RI655 to the negative pole of the 48 volt exchange battery. Accordingly, the series circuit above traced is normally completed, whereby the cathode heaters heat the associated cathodes of the tubes mentioned rendering them electron emissive and effecting operation of the alarm relay RI655. Of course, it will be understood that in the event any one of the tubes mentioned is removed from its socket or develops a burned out or broken cathode heater, the above traced series circuit is interrupted, causing the alarm relay RI655 to restore in order to complete, at the contacts I656, a circuit including the contacts I6I8 of the key K2 for illuminating a cathode heaters alarm lamp LI65I, thereby to indicate the failure mentioned. Moreover, the potential drop along the above-traced series cathode heater circuit is employed at various points for grid biasing purposes, explained more fully hereinafter; and, in passing, it is noted that a negative potential of 6.3 volts appears upon the conductor CI6I3, a negative potential of 18.9 volts appears upon the conductor CI6'I5, and a negative potential of 20.1 volts appears upon the conductor CIE'IB.

The tube TI is employed as a two-way amplifier, being utilized as an outgoing amplifier and modulator in the transmission of signals to the associated repeater circuit. The screen grid of the tube TI is supplied with positive potential from a 130 volt source of plate current, the negative pole of which is grounded, through an associated 50,000 ohm resistor I629 and is by-passed to the cathode of the tube Tl by a 0.1 mf. condenser I630 connected therebetween; and the suppressor grid of the tube TI, employed as a modulation grid, is connected either to ground potential or to the 60 cycle conductor .CI6I2, depending upon the operated or restored condition of the modulation cutoff relay RI620 and the receiving switch relay R1640 as explained more fully hereinafter.

10 For the reception of signals, the primary winding of the receiving transformer TRI is connected to the receiving conductor Cl605 and to the receiving conductor CI606 through the two illustrated 0.1 mi. condensers; the secondary winding of the receiving transformer TRI is normally connected by way of the contacts I64l of the normally operated receiving switch relay RI640 to the control grid of the tube TI in order to control the potential applied thereto; a negative potential of 20.1 volts being normally applied to the control grid of the tube Tl from the conductor CIBIB by way of a 0.15 megohm resistor R2, the secondary winding of the receiving transformer TRI and the contacts [64V of the receiving switch relay RI640. The cathode of the tube TI is normally connected by way of the contacts I643 of the normally operated receiving switch relay RI 640 to the conductor CI6'I5 having negative 18.9 volt potential applied thereto. Accordingly, the control grid of the tube Ti has a negative biasing potential of 1.2 volts applied thereto with respect to the cathode by virtue of the 1.2 volt drop across the resistor RI.

With the receiving switchrelay Rli'i til in its normal operated position, the suppressor grid of the tube TI is connected by way of the contacts I606 thereof to ground potential, as previously noted; and the secondary winding of the sending and coupling transformer TRZ is connected, at the contacts I642 and I644 of the operated receiving switch relay-RI 640, across the leads CI625 and CI626. The primary winding of the monitoring transformer TR3 is, connected by way of the two 5000 ohm resistors i623 and i620 across the leads C1625 and CIBZB; while the secondary winding of the monitoring transformer TRB is connected directly to the monitoring conductor (H603 and the monitoring conductor CIBM extending to the associated repeater circuit. Thus, audible signals amplified by the tube TI andimpressed through the sending and coupling transformer TR2 across the leads CI625 and C I 626 are supplied through the monitoring transformer TR3 to the monitoring and monitoring conductors CI603 and Ci664 extending to the associated repeater circuit, for a purpose more fullyexplained hereinafter. Further, the plate of the tube TI is connected by way of the associated 6500 ohm plate impedance I628 to the positive terminal of the volt sourceof plate current supply, and is also connected by way of a 0.04 mi. coupling condenser I 62'? to one terminal of the primary winding of the sending and coupling transformer TRZ, the other terminal of theprimary winding mentioned being grounded. An inductance coil L3 is connected across the leads CI625 and (H620 in series with a 0.04 mi. tuning condenser I631, which condenser is shunted by the indicated trimmer condenser of such capacity as to render the couple seriesresonant at 1000 cycles. Finally, the junction between the inductance coil L6 and the tuning condenser I63I is connected to the right-hand plate of the tube T2, whereby a relatively high potential is placed on the right-hand plate of the tube T2 by the voltage drop across the inductance coil L3 when a 1000 cycle current is being received and amplifiedby the tube Ti. In order to enable the system to respond to impulses of 1000 cycles plus or minus an amount equal to about 60 cycles, so as to allow for the reception of 60 cycle modulated, 1000 cycle pulses, and to cut off sharply when frequencies higher or lower than the allow- 

